The present invention relates to a magnetic field source measuring method and system in which a magnetic field distribution on an external surface of an object such as a living body to be examined is measured to determine the distribution of magnetic field generation sources in the interior of the object from data of the measured magnetic field distribution and the determined magnetic field generation source distribution is displayed.
There are known techniques for determining a current distribution in an object such as a living body from a magnetic field distribution on an external surface of the object. Since the current distribution in the object and the magnetic field distribution on the surface of the object are related with each other by the Biot-Savart's equation, the current distribution in the object can be determined by measuring the magnetic field distribution on the object surface and discretely performing an inverse operation of the Biot-Savart's equation.
The current distribution thus determined includes an impressed current in which an electromotive force attendant upon the activity of the living body is directly reflected and a return current which is subsidiarily generated from the impressed current. Accordingly, in order to accurately visualize the activity of the living body, it is necessary to remove a return current component from a current distribution which is obtained from the inverse operation of the Biot-Savart's equation (hereinafter referred to as total current distribution).
One example of a method of removing the return current component has been proposed by, for example, W. H. Kullmann, SPIE Proceedings, Vol. 1351, pp. 399-409. According to the proposed method, when the distribution S of electric conductivities in a living body is uniform, the following expression is satisfied owing to the continuity of current: EQU S.gradient.2V =-.gradient..multidot.P (1)
where V is a potential in the living body and P is a three-dimensional impressed current vector. The expression (1) is the Poisson's equation. The potential V is determined by solving the expression (1) with an impressed current being supposed at each pixel position. In the expression (1), .gradient. is a symbol for differentiation. Next, the distribution of return current vectors R corresponding to the position of the impressed current is determined from EQU R=-S.gradient..multidot.V.multidot. (2)
Further, a return current component is determined utilizing a linear relationship satisfied between the impressed current and the return current, and the determined return current component is removed from a total current.
As mentioned above, a return current must be removed in the case where an impressed current distribution in a living body is reconstructed from a magnetic field distribution measured in the exterior of the living body. However, the Kullmann's method involves a problem that an error is included since conductivity is considered as uniform and a problem that a large amount of computation time is required since the Poisson's equation is solved a number of times which is equal to the number of pixels multiplied by 3. Therefore, the practicability also comes into question.